My experience with the VP100 and 150 is that they are virtually the same as far as bass extension, the 150 only holds a marginal edge over the 100, really not alot of difference and it was tough to hear any difference as well. The bigger difference is the 150 will play louder. Either way after owning a VP180 I couldn't imagine going back to something with less FR for a center, luckily I do not own one, only borrowed one for a week.
I highly doubt you will find anything with small size that will work as well, best you can hope for is something less wide and taller like the VP160 should be. Its all about cabinet size and driver surface area and small for either one doesn't do it.
I would recommend the Paradigm Studio series centers although not what I would call small the CC590 is not huge nor overly tall and has good bass extension. IMO, it sounds very good, but does come with a pretty good price tag.
